Interactions in the Brassica napus-Pyrenopeziza brassicae pathosystem and sources of resistance to P. brassicae (light leaf spot)

Pyrenopeziza brassicae, cause of light leaf spot (LLS), is an important pathogen of oilseed rape and vegetable brassicas and has a wide geographic distribution. Exploitation of host resistance remains the most sustainable and economically viable solution for disease management. This study evaluated 18 oilseed rape cultivars or breeding lines for host resistance against P. brassicae in glasshouse experiments. Selected cultivars/lines were inoculated with eight single‐spore isolates of the pathogen obtained from three different regions in England. Analysis of P. brassicae infection‐related changes on host plants identified leaf deformation as a characteristic feature associated with P. brassicae infection, this showed poor correlation to LLS severity measured as the amount of pathogen sporulation on infected plants. Resistant host phenotypes were identified by limitation of P. brassicae sporulation, with or without the presence of a necrotic response (black flecking phenotype). Investigation of this pathosystem revealed significant differences between cultivars/lines, between isolates, and significant cultivar/line‐by‐isolate interactions. In total, 37 resistant and 16 moderately resistant interactions were identified from 144 cultivar/line‐by‐isolate interactions using statistical methods. Most of the resistant/moderately resistant interactions identified in this study appeared to be nonspecific towards the isolates tested. Our results suggested the presence of isolate‐specific resistant interactions for some cultivars. Several sources of resistance have been identified that are valuable for oilseed rape breeding programmes.